The objectives of this proposed research are: to determine the kinetics and extent of repair of double-strand breaks in DNA and the molecular mechanisms involved in such repair; to determine the effects of increasing doses of ionizing radiation on the capability of cells to repair double-strand breaks in DNA; to establish the effect of DNA double-strand breaks induced by ionizing radiation on cell survival. These studies will be done in bacteria, yeast and mammalian cells using sedimentation and fluctuation spectroscopy to measure DNA molecular weight and thereby numbers of double-strand breaks and repair of these breaks.